An in-wheel motor unit (a wheel driving apparatus) configured to drive a wheel by a motor disposed inside the wheel has conventionally been known as one form of an electric automobile (refer Japanese Patent Application Laid-Open (kokai) No. 2016-073061.).
This in-wheel motor unit comprises the motor and a speed reducer configured to change (increase) torque of the motor to output. The speed reducer includes a rotating body (for example, a counter gear) configured to be rotated by the motor, and an output shaft fixed to the rotating body. The output shaft extends to outside of a housing in an axle direction via an opening of a wall surface of the housing. The output shaft is rotatably supported with respect to the housing by a hub bearing comprising a bearing part and a hub part. When the motor is supplied with electric power, torque of the motor is changed (increased) via the speed reducer, and the changed (increased) torque is transmitted to the wheel fixed to the hub part of the hub bearing, thereby the wheel is rotated. Hereinafter, the in-wheel motor unit will be also simply referred to as a “motor unit”.